There is no where in the written rules on the page linked "rules" that joint attacks are illegal. The point of this game is to win, and destroying your enemies is one way to accomplish that. Considering that it's not written in the rules, I'd also think a warning before country destruction would be warranted.

Mod: I originally thought you killed my country because I may have been logging in from several different locations, that's the only reason I brought up the VPN.

Have a nice day